In many rotary machines, such as a multi-stage centrifugal compressor or pump, a fluid is compressed by successive stages, or in turbines, a fluid is expanded in successive stages. Both turbine and compressor stage(s) have stationary or non-rotating components, e.g., vanes, cooperating with rotating components, e.g., blades, for compressing and expanding the operational fluid. The operational fluids change in pressure through the machine and a variety of seals are provided to preserve the differential pressures where necessary to maximize machine efficiency and performance. An illustrative seal may be provided between a turbine or compressor rotor and a cooperating stator or stator body so the rotor may be pressurized to provide thrust balance relative to the rearwardly directed force generated by the equipment and the forward direction of the equipment.
In the above-described settings, the seals used must address the close operating clearances required in machinery of this type. Rotary machine seal design also requires consideration of the relative motion between components produced by the differential thermal expansion and system pressure that occurs throughout the machinery operating cycle compared to shutdown clearance at assembly and transient rotor dynamic displacements traversing critical shaft speeds.
One structure commonly provided to control leakage flow along a turbine shaft or other rotating surface is a labyrinth seal. In this setting, a variety of blocking seal strips and obstructions are used between stationary turbine components. Solid labyrinth seals typically have a relatively large clearance to avoid rub damage. Labyrinth seals, therefore, do not maximize machine performance.
Another commonly used seal is a brush seal, which includes a pack of metal bristles that contact a rotor at free ends thereof to maintain a seal with the rotor. Brush seals have some resilience to accommodate rubbing against the rotating component. For instance, in U.S. Pat. No. 5,090,710, issued to Flower, a brush seal is comprised of closely packed fine wires or filaments that are weld assembled in a carrier assembly that is then inserted in a machine with the bristles wiping the rotating surface. The bristles and assembly are fabricated of materials suitable for the fluid temperature and, compared to a labyrinth seal, leakage is reduced through and past the bristles in close contact with the rotating surface.
Brush seals, however, pose a number of deficiencies. First, the multistep brush seal manufacturing process is costly. Second, brush seal bristles do not always maintain a close running clearance because of their inherent inability to withstand long term wear. Third, brush seals exposed to solid particles are subject to erosion or other deterioration. Finally, brush seals are also subject to vibration due to movement of the pressurized fluid being sealed. Therefore, brush seals oftentimes require dampening features.
According to U.S. Pat. Nos. 6,428,009, 7,182,345 and 7,410,173 issued to Justak and U.S. Pat. No. 5,026,252 issued to Hoffelner, brush seal designs with hydrodynamic shoes attached under the bristle ends of seal to reduce seal leakage are provided. Various arrangements and methods of attachment of brush seal components and hydrodynamic slider members are disclosed in these patents, however, robust integration and improved function is needed to achieve the reduced seal leakage objective of a hydrodynamic shaft seal. A particular deficiency in these designs is initial contact of the hydrodynamic shoes with the shaft until sufficient rotor speed is achieved during startup to produce the requisite film thickness to lift the shoes from rubbing contact with the shaft.
Another type seal is a finger seal, for example, those disclosed in U.S. Pat. Nos. 5,042,823 and 5,071,138, both issued to Mackay et al. These disclosures disclose a laminated finger seal providing a planar array of radially and circumferentially extending fingers separated by gaps. This structure suffers from a number of disadvantages. For instance, each stacked lamination is a complete ring (not segmented) and, therefore, is limited in application to machines that require installation/replacement of seals with the rotor removed from the unit.
In addition to the above-identified problems, brush seals and finger seals operating at close running clearance are subject to rubbing and wear when differential thermal expansion of stator and rotor components eliminates clearance altogether. For example, during a turbine shutdown, the stator component, in which a seal assembly is mounted, may cool more quickly than the rotor, causing the seal assembly to close on the rotor and rub. The force imposed during such a rub is reduced somewhat with the flexure of brush and finger seal members, but sliding friction nevertheless causes wear and reduces the life of such seals. Another example, in the aerospace area, is differential seal pressures at the compressor discharge of in aircraft engines during take-off can be three to four times higher than at idling or cruise conditions. High rotor speed and engine temperature during take-off increases the diameter of rotor seal surfaces, closing seal clearance and raises the opportunity for rubbing and wear.
One type of seal that addresses some of the deficiencies noted above for labyrinth, brush and finger seals is a leaf seal. Leaf seals are used to create a non-hermetic seal between abutting structural components in a turbo machine or other apparatus wherein a high pressure area is present on one side of the structural components and a low pressure area is present on the opposite side thereof. Leaf seals are typically relatively thin, compliant sections which may be manufactured to include narrow, precision slots to produce the desired seal member geometry. Where the structural components to be sealed are annular in shape, as in many components of turbo machines, segmented leaf seals are employed, i.e., relatively short, arcuate-shaped leaf seals which abut one another to form an essentially continuous annular seal between the structural components.
Regardless of the particular shape of the structural components to be sealed, leaf seals are movable between a closed, sealing position in which they engage (but not necessarily contact) each structural component and seal the space therebetween, and an open position in which at least one portion of the leaf seals disengage a structural component and allow the passage of gases in between such components.
An example of a seal assembly including leaf seals can be found in U.S. Pat. No. 7,578,509, and U.S. patent application Ser. No. 12/632,224, which are incorporated herein by reference. These patents disclose a plurality of metallic leaf seal members with each leaf seal member angled between their free ends and their fixed ends and a support member for supporting the angle. Seal member geometry is engineered with respect to thickness, width, length, and number of members to meet specific application requirements of differential pressure and anticipated differential motion. The support member serves to limit leaf seal member movement in one direction and withstand differential pressure, while a force imposed by a rubbing engagement on a rotating component is reduced with the elastic flexure of the seal assembly. Seal member end geometry may be shaped to provide a precision diameter.